In conventional electrophotography, a photoconductive surface is charged in the dark and then subjected to a light image of the document or photograph which is to be reproduced, generating a latent electrostatic image corresponding to the original document or photograph. The latent electrostatic image is then made visible by toning with electroscopic particles. The most widely used photoconductor in electrophotographic machines is a vitreous or, more commonly called, amorphous selenium. Its sensitivity, however, is chiefly in the ranges of blue to yellow, and the gray scale is such that copies of photographs are very poor. Furthermore, a selenium photoconductor cannot be heated to a temperature of over 80.degree. C. without losing its electrophotographic properties. The chief disadvantage of a selenium photoconductor is that it wears rapidly in an electrophotographic machine and must be replaced after use for between ten thousand and one hundred thousand copies. The addition of tellurium, arsenic, and other dopants to amorphous selenium is known in the art to effect some improvement on these properties, but substantial improvement is desirable, particularly in the wear characteristics.
Cadmium sulphide has a hardness of between 3 and 3.5 on Moh's scale of hardness. Its spectral response, when properly formed and doped, is across the entire visible range from blue to red. It has a higher effective quantum efficiency--that is, the ability to convert light into charge--from twice to ten times that of selenium. Its light discharge characteristic is such that it produces an excellent gray scale, enabling it to make excellent reproductions of photographs.
A photoconductor, in an electrophotographic process, is mounted on a conductive substrate and charged by a charging corona. The corona ionizes the air. This ionized air acts as one plate of a capacitor, the other plate being the conductive substrate. Since a photoconductor is a dielectric in the dark, the charge from the corona sticks to the surface of the photoconductor. This induces a charge of opposite polarity on the conductive substrate. The time it takes to tone a latent electrostatic image on the surface of a photoconductor is dependent on the voltage to which it may be charged.
The prior art has recognized the advantages of polycrystalline cadmium sulphide as a photoconductor. Unfortunately, the prior art (Kuehnle U.S. Pat. No. 3,884,787) has been unable to form a photoconductor of cadmium sulphide of adequate thickness to create a sufficiently high voltage at the surface with the required charge densities. This means that the development of a latent electrostatic image produced on the surface of the photoconductor will take an inordinately long time for practical use. When it is attempted to make a photoconductor of cadmium sulphide thick, it flakes from the conductive substrate or cracks.
Corrsin U.S. Pat. No. 3,151,982 attempted to overcome the short life of a vitreous selenium photoconductor by using cadmium sulphide particles in a glass binder. Lane U.S. Pat. No. 3,510,298 also discloses a cadmium sulphide photoconductor in a glass binder. We have found that glass-bound cadmium sulphide does not produce a commercially usable electrophotographic photoconductor. The latent electrostatic images, when developed, were full of spots which spoiled the images.
The prior art had developed two techniques for obtaining cadmium sulphide photoconductors without the use of binders. The first of these is described by Hill et al in U.S. Pat. No. 3,148,084. The prior art, in respect of obtaining photoconductive films, is discussed in this patent, and the disadvantages of the evaporation process, the chemical deposition process, and the vapor-reaction process are pointed out. Hill et al disclose the formation of photoconductive films by spraying reagents on a heated substrate. Their films include sulphides of many metals, as well as sulphoselenides of cadmium, cobalt, and indium. The photoconductive films were formed on an insulating substrate. Co-inventor Chamberlin further described the method in the Journal of the Electrochemical Society, Volume 113, pages 86-89, in an article written with J. S. Skarman in 1966. The films were not intended to be used for electrophotography, but, rather, in the manufacture of thin-film solar cells. These photovoltaic converters were formed by a thin film of copper sulphide (0.1.mu.) together with a thin film of cadmium sulphide (1.mu.).
Another method of forming thin-film photoconductors is by sputtering. This process is described by Kuehnle in U.S. Pat. No. 3,884,787. Films having a thickness up to 0.5.mu. (5000 A) were formed. These films were transparent to yellow light and were excellent photoconductors.
A charged photoconductor in the dark is analogous to a charged capacitor in which the photoconductor is the insulating or dielectric medium. In order to achieve a rapid development, a high voltage is necessary to attract toner particles from a large distance. In the case of cadmium sulphide, the corona charge is negative, so the charge of the toner particles of the developer is positive. The speed at which the charged toner particles in the developing liquid move to the latent electrostatic image on the photoconductor is a function of the voltage of the latent electrostatic image. The higher the voltage, the speedier will be the development. With a thin-film photoconductor, a surface charge density above a certain value cannot be maintained, and the excess charge is transported across the dielectric. The voltage generated at the maximum charge level is proportional to the thickness of the photoconductive layer and inversely proportional to the dielectric constant. Thus, in order to provide the high voltages required for fast toning, we seek to increase the charge density accepted by the surface and to make the film as thick as possible. When we attempted to form a thicker film by carrying on the pyrolytic formation of cadmium sulphide from aqueous solutions of reagents to a greater extent, the film flaked off the conductive substrate.
Shattuck et al U.S. Pat. No. 3,676,210 discloses a recognition of the defects in Hill et al 3,148,084, for use as an electrophotographic photoconductor, and attempts to overcome these disadvantages of a thin film by using a resin binder. In Shattuck et al, the inventors use an aqueous emulsion of polyvinyl acetate in the method disclosed by Hill et al and obtain a resin-bound cadmium sulphide photoconductor. There is no disclosure of the use of zinc or of copper as dopants. Reference is made, in Shattuck et al, to Middleton et al U.S. Pat. Nos. 3,121,006 and 3,121,007, both of which disclose a photosensitive layer consisting of an inorganic photoconductive powder dispersed in a resin binder. Shattuck et al merely use the Hill et al process to manufacture the Middleton et al products. Shattuck et al set forth that the photoconductive compounds which are formed by their method are not crystalline, but are, instead, amorphous.
Cadmium sulphide, as an electrophotographic photoconductor, has several disadvantages. First, it has a memory; that is, after charging and exposing to a light image to form a latent electrostatic image, developing that image, and transferring it to a carrier sheet, the latent image still remains on the photoconductor. Stated otherwise, the decay time in the presence of light is too slight, so that offsetting occurs. Another disadvantage of a cadmium sulphide electrophotographic photoconductor is fatigue; that is, as the photoconductor is used and reused, the maximum voltage to which it can be charged becomes less and less. Cadmium sulphide, however, can retain a much higher charge density than tellurium-doped selenium. Cadmium sulphide has a higher photosensitivity than selenium.